Martian Moons Have a Strange Cycle of Existence

Researchers suggest that Martian moons may someday break apart and form a ring around the planet. Then, eventually, these fragments may come together to form a moon once again.

The moons Phobos and Deimos have always been a little strange. For one thing, they don't exactly look like moons, but they behave like and actually are moons. A recent study now suggests that these moons may have been been part of a cycle of ring- and moon-building around Mars.

There was a suggestion that Phobos and Deimos were once asteroids that wandered too close to Mars. This may certainly explain their asteroid-like appearance, but it doesn't explain much else about the two Martian moons. First, asteroids don't usually wander that close to Mars, and if they did, they would not have a neat orbit around the planet. In fact, their orbits would probably go against the rotation of the planet.

However, both Phobos and Deimos have a neat orbit around the planet's equator. They also traverse their orbits along the planet's rotation, which suggests that the moons formed in that orbit.

So how did these moons form, and how are they destined to break apart and form a ring around Mars?

The Life Cycle of Martian Moons

Scientists say that Phobos, the larger of the two moons, is orbiting closer and closer to Mars. Eventually, it will get close enough to crash into the planet.

A massive impact on the surface of Mars may have formed the two moons, much like the origin of Earth's moon. This impact may happened when an asteroid or some other celestial body slammed into Mars about 4.3 billion years ago. According to astronomers Andrew Hesselbrock and David Minton, the debris from the impact formed a ring around Mars. Most of the debris ended up falling back to the planet's surface, but some managed to spread out farther. These remainders eventually became moons.

Of course, since this is a cycle, these moons will not remain moons forever. According to the aforementioned model, gravitational interactions with Mars will eventually pull the moons back to the surface and cause a collision. The impact will again cause the formation of debris, which will again form a ring around Mars. The remaining debris will form Martian moons once again, which will eventually repeat the cycle of ring and moon formation.

Phobos Reborn

Hesselbrock and Minton postulate that this cycle has already happened three to seven times over the past billions of years. As shown above, most of the debris ends up back on Mars. This means that each moon formed in every cycle is smaller than its past incarnation.

Phobos gets closer to Mars by about two meters every hundred years. In about 70 million years, Phobos will crash into Mars's surface and break apart into debris that will form a ring around the planet. When Phobos puts itself back together again, it will be smaller than it is now.